Systems for preventing excessive discharge of such batteries are known in which the current drawn from the battery is limited after the voltage across the battery has dropped to a predetermined minimum voltage level. This predetermined minimum voltage level is set by the manufacturer and, may, for example be a final discharge voltage U.sub.ES of 1.4 volts per cell after one-half hour of discharge. If the control is carried out in such a manner that this voltage U.sub.ES is maintained without taking into consideration what current the battery can still deliver then excessive discharge substantially decreases the life expectancy of the battery, that is the number of possible discharge cycles.
German publication DT-OS No. 2,324,305 describes a control system for the battery of a DC powered vehicle which a constant maximum available power is assured over a wide discharge region of the battery by limiting the available power when the charge on the battery is high. For this purpose, a voltage proportional to the voltage across the armature is applied across the rheostat which determines the current through the armature. A signal which corresponds to the product of voltage across the armature multiplied by current through the armature is applied to a limiter stage whose action is initiated in dependence on the charge state of the battery. Although the battery is used to better advantage by means of this constant power regulation, the prior art system has the disadvantage that low voltage regulation still takes place on the basis of constant battery voltage output.